1. Field of the Invention
This invention relates to methods and systems for protecting magnetoresistive (MR) disk drive heads from electrostatic discharge (ESD) and electric overstress (EOS) events.
2. Description of the Prior Art and Related Information
Magnetic disk drives, such as those used for mass storage in personal computer systems are well known. Generally, disk drives include a head disk assembly (HDA) and a controller printed circuit board (PCBA). The HDA includes a cover, a base, one or more disks, a head stack assembly (HSA) that includes an actuator assembly, head gimbal assembly (HGA) and a flex circuit. The HSA rotatably positions a head having one or more transducers over a disk. A spindle motor spins the disks.
In older inductive heads, a single transducer performs both the reading and the writing functions. In the currently industry-favored magnetoresistive heads, the slider supports separate read and write transducers. MR heads enable higher densities because of their higher sensitivities to magnetic flux. Due to their relatively small physical dimensions, however, MR heads are particularly sensitive to ESD and EOS events. The fundamental mechanism of ESD is tribocharging, which may occur when different materials come in contact with one another and then separate, such as the sole of a person's shoe walking across a carpet, or when an IC or other sensitive element (such as an MR head) slides against or comes in contact with a metal component of an automatic manufacturing equipment. The magnitude of the generated charge is dependent upon many factors including, for example, the type of materials, the speed with the different materials slide together as well as the relative humidity of the environment in which the contact occurred.
It is to be expected that as recording densities increase and MR heads are further reduced in size, they will become even more sensitive and susceptible to damage from ESD and/or EOS events. Moreover, there are many occasions during the disk drive manufacturing process in which MR heads may become damaged. These include during the HGA, HSA and HDA manufacturing processes, in addition to damage during in-drive operation. Protecting MR heads at all stages of the disk drive manufacturing process becomes critical for maintaining high head yields, drive yields and head and drive reliability.
From the foregoing, it may be appreciated that there is a clear need for protecting the MR heads of a disk drive, at all stages of manufacture of the constituent assemblies thereof and during operation of the drive itself.
Conventional attempts to protect the heads from electrostatic discharges and electric overstress events included factory ESD precautions, resistive shunting and placing diodes in the drive's preamplifier. The increasing sensitivities of ever shrinking MR heads are quickly rendering even the most careful ESD precautions ineffective and damage to MR heads are lowering yields and causing reliability issues. Resistive shunting ESD/EOS protection schemes range from permanent shorting to adjustable shorting. Permanent shorting is effective, as it shorts out the MR heads until the short is removed. U.S. Pat. Nos. 6,400,534 and 6,424,505 are examples of such permanent shorting schemes. However, using this technique, deshunting is necessary before the HGA and HSA can be tested, after which the heads must be re-shorted. However, it may be appreciated that the very act of deshunting and reshunting the heads may itself result additional ESD events. Adjustable shunting and deshunting schemes are disclosed in U.S. Pat. Nos. 5,644,454, and 5,757,590 and require dedicated mechanisms and control circuitry to actively shunt and deshunt. Placing diodes in the preamplifier may protect the HSA after it is built, but does not provide any protection for the heads during the HGA and HSA fabrication processes. Moreover, placing diodes in the preamp is wholly ineffective to protect the heads before the preamplifier is connected to the HSA. In addition, the distance between the preamp and the heads is believed to be too great to provide effective ESD protection, as electrostatic discharges may damage the heads even before reaching the diodes.
What is needed, therefore, is effective ESD and EOS protection for the heads of a disk drive at all stages of the construction thereof. What is also needed is ESD/EOS protection that does not require deshunting prior to testing or use of the drive.